The present invention relates to a 2-cyanoacrylate composition which has a high curing rate and is excellent in moist-heat resistance, surface curability, and clearance curability.
2-Cyanoacrylates undergo instantaneous anionic polymerization to exhibit a high bonding strength. The 2-cyanoacrylates are hence widely utilized as instantaneous adhesives or rapid-setting filling materials in various industrial fields, the medical field, and the leisure field and in domestic applications. However, since the curing is based on anionic polymerization, the polymerization may be inhibited when the 2-cyanoacrylates are applied to woods or adherends which are acidic having a surface on which an oxide film is apt to form. In such cases, a prolonged setting time is required and the resultant bonding strength is not entirely sufficient. Another problem is that since the polymerization is initiated mainly by the water present in a slight amount on the adherends, a prolonged setting time is necessary on engineering plastics having a surface with a poor affinity for water, such as polyacetals and poly (butylene terephthalate).
Furthermore, use of 2-cyanoacrylates as a potting material, filling material, or instantaneous adhesive has had a problem that the composition has poor curability, i.e., necessitates a prolonged setting time, when applied to a relatively wide gap between adherends (hereinafter, curability in this application is referred to as xe2x80x9cclearance curabilityxe2x80x9d). Another problem thereof is that those parts of the composition which have overflowed on a surface necessitate a prolonged setting time and, in some cases, impair the appearance of the product (hereinafter, those properties are inclusively referred to as xe2x80x9csurface curabilityxe2x80x9d; in particular, the time required for setting is referred to as xe2x80x9csurface setting timexe2x80x9d and the appearance of the resultant cured material is referred to as xe2x80x9cappearance of surface-cured materialxe2x80x9d).
Various curing accelerators have conventionally been investigated in order to overcome such problems. For example, U.S. Pat. No. 4,171,416 discloses the use of a crown ether compound as a curing accelerator. U.S. Pat. Nos. 4,170,585 and 4,377,490 disclose the use of a polyalkylene glycol derivative having a specific structure and a mixture of an aliphatic or aromatic polyol with an aliphatic or aromatic polyether, respectively, as a curing accelerator. Furthermore, U.S. Pat. Nos. 4,386,193 and 5,589,554 disclose the use of a three- or four-arm polyol podand compound having a specific structure and a compound having cyano and carboxyl groups, respectively, as a curing accelerator.
However, 2-cyanoacrylate compositions containing those curing accelerators have a drawback that when the compositions are used as adhesives or filling materials, the resultant cured objects have impaired moist-heat resistance although an improved curing rate is attained.
Besides having such impaired moist-heat resistance, the 2-cyanoacrylate compositions containing a crown ether or polyethylene glycol derivative as a curing accelerator have a drawback that the cured material itself obtained therefrom has a milk-white surface.
The term xe2x80x9cmoist-heat resistance of a cured objectxe2x80x9d as used herein means the moist-heat resistance of a structure obtained through bonding or filling with a 2-cyanoacrylate composition. This property is a basic performance always required of cured objects for use in industrial fields including the electrical/electronic, construction/civil engineering, and automobile industries. The persons who use cyanoacrylate adhesives well know that structures formed through bonding or filling with a 2-cyanoacrylate composition deteriorate in strength with the lapse of time when used or stored at a temperature and humidity which are high in some degree, and finally become unsuitable for practical use. This phenomenon is described in known documents including Kxc3x4gyxc3x4 Zairyxc3x4, 16(12) 73 (1968).
Furthermore, the term xe2x80x9cmilk-white appearance of a cured materialxe2x80x9d as used herein means that a cured material formed by applying a 2-cyanoacrylate composition to a substrate and curing it in air appears milk-white. It is thought that when such a 2-cyanoaccrylate adhesive is used, the overflow which can result from application of the adhesive does not remain colorless and transparent but becomes white to thereby impair the appearance of the product.
The present inventors found that use of the conventional 2-cyanoacrylate compositions whose curability has been improved with curing accelerators which are added to 2-cyanoacrylate composition results in a considerable decrease in moist-heat resistance. As will be demonstrated by the Comparative Examples given later, when 2-cyanoacrylate compositions containing a crown ether, polyethylene glycol derivative, or Calixarene derivative as a curing accelerator are used to bond or fill various materials, the cured objects thus obtained have lower moist-heat resistance than cured objects obtained with 2-cyanoacrylate compositions containing none of these curing accelerators. In the case where a curing accelerator is incorporated in a larger amount so as to obtain an even higher curing rate, the decrease in moist-heat resistance becomes greater. Conversely, in the case where a curing accelerator is added in a reduced amount so as to prevent deterioration in strength and attain satisfactory moist-heat resistance, the instantaneously bonding properties of this composition is on an unsatisfactory level.
Consequently, it has been necessary that a 2-cyanoacrylate composition containing a reduced amount of a curing accelerator and having reduced curability be selected for use in applications wherein moist-heat resistance is important and that the insufficient curability thereof be compensated for by heating the adherend, using a primer, etc. This technique, however, considerably impairs applicability, which is a merit of 2-cyanoacrylate compositions. Although there is a desire for a rapid-setting 2-cyanoacrylate composition having high moist-heat resistance, it has been difficult to satisfy this requirement with any conventional curing accelerator.
Many attempts have been made to enhance moist-heat resistance. For example, JP-A-7-33726 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-A-6-145605 disclose a process for producing a polyfunctional cyanoacrylate and the incorporation of a polyfunctional acrylate having a specific structure, respectively. JP-B-56-54029 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) and JP-A-48-103633 disclose the incorporation of a carboxylic anhydride and the incorporation of a specific organic silane compound, respectively. JP-A-57-164173 and JP-A-2-208380 disclose the incorporation of a 1,1-disubstituted diene and the incorporation of an ethoxysilane isocyanate into a 2-cyanoacrylate just before use, respectively. JP-A-3-265675 and JP-A-4-146982 disclose the incorporation of a specific phosphoric acid compound and the incorporation of a specific silicone oil, respectively. JP-A-2-311583 and JP-A-6-57215 disclose the incorporation of fine particles of a crosslinked polymer synthesized by the non-aqueous dispersion method and the incorporation of a specific polyhydroxy compound and an acrylic ester copolymer elastomer, respectively. Furthermore, JP-A-7-53924 and JP-A-7-157719 disclose the incorporation of an acrylonitrile/butadiene copolymer and a specific amount of pyrogallol and the incorporation of a specific amount of a phthalic ester, respectively. However, each of those techniques has only given results which are unsatisfactory in curability.
On the other hand, it has been proposed to use various surface-treating agents for the purpose of improving the clearance curability and surface curability of 2-cyanoacrylate compositions. For example, JP-B-62-29471 discloses a composition comprising an N,N-di(lower alkyl)toluidine, saccharin, and ascorbic or isoascorbic acid, and JP-B-62-12279 discloses the use of a specific surfactant. JP-B-60-24826 discloses the use of methylolurea which may have been partly alkoxylated, melamine, or a derivative of either. JP-B-54-19416 and JP-B-51-25441 disclose a technique in which an adherend is treated beforehand with N, N-dimethyl-m-toluidine and with a vinyl ether or vinyl thioether, respectively. JP-B-49-12094 discloses a technique in which an adherend surface is treated with a liquid containing an amine, amide, or imide, and JP-B-48-44175 discloses a technique in which an adherend surface is treated with a liquid containing a specific heterocyclic compound. JP-B-47-8718 discloses a technique in which an adherend surface is treated beforehand with a liquid containing dimethylaniline or/and tris[1-(2-methyl)aziridiyl]phosphine oxide. Furthermore, JP-A-39-844 discloses a technique in which an adherend surface is treated beforehand with an epoxide or diepoxide.
Curing accelerators to be added to 2-cyanoacrylate compositions have also been investigated in order to improve clearance curability and surface curability. For example, JP-B-5-72946 and JP-A-7-316505 disclose the use of a Calixarene compound and an organotitanium compound, respectively, as a curing accelerator.
It has further been attempted to improve surface curability and clearance curability by imparting photocurability to a 2-cyanoacrylate composition. For example, JP-A-9-249708 discloses the use as an anionic photopolymerization initiator of a metallocene compound comprising a transition metal of Group VIII of the periodic table and aromatic ligands. JP-A-6-299122 and International Publication WO 93/10483 disclose the use of an aromatic azide compound and an inorganic chromium complex, respectively, as an anionic photopolymerization initiator. Furthermore, JP-A-62-57475 discloses a cyanoacrylate-based adhesive composition containing a free-radical photopolymerization initiator.
However, the techniques described above each have had one or more drawbacks as will be described below, such as poor applicability, poor storage stability, and the necessity of a light irradiator. Namely, those techniques have been unsatisfactory because the advantage inherent in 2-cyanoacrylate compositions that the operation can be completed in a short period of time is lost.
Specifically, those conventional techniques have the following drawbacks. The technique in which a surface-treating agent is used is satisfactory in attaining a shortened operation period because it greatly improves surface curability and clearance curability. However, applicability is considerably impaired because two liquids should be applied.
The technique in which a curing accelerator is used is effective in improving curability in application to closely spaced adherends. However, this technique has been unsatisfactory in surface curability and clearance curability.
The technique in which photocurability is imparted is satisfactory in operation period because it can improve surface curability and clearance durability. However, this technique has been unsatisfactory in that it necessitates a light irradiator and that it is unusable for the bonding of adherends having such a structure that irradiation of the bonding surface with light is impossible. Namely, there has been a desire since long before for a 2-cyanoacrylate composition which consists of one component curable at ambient temperature and not necessitating a light irradiator or the like and which cures in a short period of time even in the case where it is used as a potting material, has overflowed, or is applied to a relatively wide gap between adherends.
The present invention has been made to overcome the above-described drawbacks in the conventional techniques.
Accordingly, an object of the invention is to provide a 2-cyanoacrylate composition which is free from a considerable decrease in moist-heat resistance and has a sufficiently high curing rate even when applied to adherends having a gap, and overflow parts of which also cure rapidly and are prevented from giving a milk-white cured material.
As a result of intensive investigations made by the present inventors in order to overcome the above-described problems, it has surprisingly been found that when a cyclic phenol sulfide derivative having a specific structure is used as a curing accelerator for a 2-cyanoacrylate, then all the problems described above are overcome. Namely, this composition was found to retain a sufficiently high curing rate without causing a decrease in moist-heat resistance, has a sufficiently high clearance curing rate, and gives a cured material having a colorless transparent appearance. The invention has been completed based on this finding.
The invention provides a 2-cyanoacrylate composition which contains a cyclic phenol sulfide derivative having a specific structure.